1. Field of the Invention
The present invention relates to a digital TV receiver and more particularly to an apparatus for judging if co-channel interference exists in a digital TV receiver using the vestigial sideband (VSB) mode.
2. Discussion of Related Art
According the digital television broadcasting standardized by the Grand Alliance (GA), both the digital and NTSC television systems use radio frequency (RF) signals. Also, each channel band of the digital and NTSC television system is limited to 6 MHz However, a significant difference between the digital and NTSC television system is in the transmission spectrum of the broadcasting.
In the transmission spectrum of the NTSC system, a video carrier frequency is always 1.25 MHz above the low end of a channel, a chrominance subcarrier frequency is 3.5 MHz above a video carrier signal, and a sound carrier signal has an unmodulated center frequency of 4.5 MHz higher than the video carrier frequency for the same channel. In contrast, the transmission spectrum of the DTV system only has a small pilot carrier signal with an unmodulated center frequency of 0.31 MHz to assist the carrier recovery at the receiver. Thus, the spectrum in the band is flat.
Moreover, DTV signals are coded and transmitted in a different manner from the coding and transmission methods of the analog signals for the NTSC system. Thus, a viewer generally cannot watch a NTSC broadcast with a DTV system. Nevertheless, a same tuner may be used since the DTV and NTSC system both use the same RF channel and an identical intermediate frequency (IF) band.
However, a co-channel interference may occur during the DTV broadcasting if an NTSC is broadcasted through a co-channel. The carrier powers, namely the video carriers, the chrominance subcarrier, and the sound carrier, of the NTSC system is significantly larger than the pilot signal of the DTV system, resulting in a severe interference in the DTV broadcasting. Generally, a comb filter is used to remove the co-channel NTSC interference.
The comb filter consists of delays and subtracters. Each delay consists of 12 resistors connected in series and which delay the received signal for 12 symbols to obtain the transmission characteristics suitable for filtering. The comb filter also subtracts the delayed data for 12 symbols from the current data. The carrier powers of the NTSC system is then removed by the comb filter by converging to xe2x80x9c0xe2x80x9d (zero). Specifically, the video carrier, chrominance carrier, and sound carrier of the NTSC system are removed because the carriers exist around the null point when passing through the comb filter, thereby avoiding interference with the DTV system.
However, using a comb filter increases the power of the random noise, resulting in a deterioration of the picture quality. Because the deterioration of the picture quality is insignificant compared to the effect of removing the NTSC channel interference, using a comb filter is still preferable. Furthermore, since the co-channel NTSC interference does not always exist, a digital TV receiver judges whether a NTSC co-channel interference exists and uses the comb filter when a co-channel NTSC interference occurs in the DTV system.
FIG. 1 is a block diagram of the DTV system. When RF signals modulated according to the VSB system are received through an antenna, a tuner 11 selects a desired channel frequency by tuning and converts the frequency into an IF signal. A frequency-phase locking loop (FPLL) 12 demodulates the IF signal from the tuner 11 into baseband signals I and Q to locks the output phase and frequency. The FPLL 12 is a circuit unifying both a frequency tracking loop and a phase-locking loop, but the frequency is locked prior to the phase.
An analog/digital (A/D) converter 13 converts the I signal into digital data of a given bit number (for example, 10 bits). The Q signal is used for carrier restoration within the FPLL 12. Using the converted digital data of a given bit number, a sync signal recovery unit 14 recovers the synchronization (sync) signal to be used for timing recovery and equalization.
The sync signal recovery unit 14 restores a data segment sync signal and a field sync signal, inserted in the RF signal by the transmission side. Namely, the standardized VSB transmission signal includes a specific sync signal patterns within the transmitted data which allows a receiver to easily recover the sync signals. Accordingly, the data incorporates consecutive data segments, wherein each segment consists of 832 symbol units and within the 832 symbol units, four symbols define the segment sync signal pattern while the other 828 symbols are data information. Also, a field consists of 313 data segments and at every 313the data segment position, a field sync signal is inserted. The segment and field sync signals are similar to a horizontal and vertical sync signals of the NTSC broadcasting signals, respectively, but do not have the same form. Because the sync signals allow an easy data recovery, an erroneous detection and recovery of the sync signals adversely affect the overall system.
An equalization and error coding correction (ECC) unit 15 uses the data segment and data field signals as training signals in an equalization. The equalization is performed to correct linear distortion of amplitude which causes interference between symbols, and to reduce ghost generated by reflection of the transmitted signal off of mountains and buildings. Afterwards, an ECC is performed to correct errors that may have occurred during transmission through the channel. A video decoder 16 decodes the equalized and error corrected signal according to the moving picture expert group (MPEG) algorithm, allowing the TV audience to watch a broadcasting through display 17.
As discussed above, a co-channel interference may occur during a DTV broadcasting. Although a comb filter may remove the co-channel interference, the use of a comb filter would increase the power of the random noise. Thus, when the co-channel interference does not occur, a use of the comb filter may unnecessarily result in a deterioration of the picture quality. Accordingly, an apparatus for judging if co-channel interference exists in a digital TV receiver is disclosed in co-pending application Ser. No. 09/207,263 entitled xe2x80x9cMethod and apparatus for determining co-channel interference in digital television system,xe2x80x9d fully incorporated herein.
In the co-pending application, a determination is made whether a co-channel NTSC interference exists before the sync recovery unit 14 outputs the recovered sync signals to the. equalization and ECC unit 15. If a co-channel NTSC interference does not exist, data bypassing a comb filter is output to the equalization and ECC unit 15. If a co-channel NTSC interference is determined to exist, a comb-filtered data is output to the equalization and ECC unit 15.
However, because terrestrial broadcasting signals such as DTV and NTSC broadcasting signals are transmitted through space, the signals are exposed to various types of noises and may be significantly attenuated by such noises. The noise which is picked up by a channel during transmission is a Gaussian noise. The power of the Gaussian noise is generally constant, but may sometimes surge to a large value. In such cases, the Gaussian noise may be determined to be a co-channel NTSC interference causing the data to pass a comb filter before being output to the equalization and error code correction unit 15. In contrast, a NTSC interference may be judged not to exist by the apparatus disclosed in the co-pending application due to the noise even if the a severe NTSC interference does exist. If data which passed or did not pass the comb filter is incorrectly selected and output, the operation of equalization and ECC unit 15 would be adversely affected. Therefore, circuits removing noisy signals contained in the incoming digital signals are affected, causing a malfunction of the DTV receiver.
Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the related art.
An object of the present invention is to provide an apparatus for judging if co-channel interference exists in a DTV receiver and varying the point in time to turn a filter for removing the NTSC co-channel interference on/off.
Another object of the present invention is to provide an apparatus for judging if co-channel interference exists in a DTV receiver, in which a Gaussian noise is not misrecognized as the NTSC co-channel interference.
A still another object of the present invention is to provide an apparatus for judging if co-channel interference exists in a DTV receiver, in which a constant size of noise is output.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, an apparatus for judging if co-channel interference exists in a DTV receiver includes obtaining the MSE of a noise which did not pass To through a comb filter, controlling its gain and comparing the MSE with a MSE of a noise which passed through a comb filter, to turn the comb filter on/off according to the compared result.